I suspect the remains on the boat are not the originals. The inside piece looks like OSB. Is it? I would not use that on a boat! The fasteners–bolts and fender washers–don’t look original either. But the shapes look reasonable, the point being to transfer loads to the sides and bottom of the boat. Treated plywood has been reported to be corrosive to aluminum. I would try to clean out the pitting with aluminum cleaner and apply etching primer.

Opinions seem to vary a lot about what sort of preservatives/coatings to use on transom wood.